1. Field of the Invention
The present invention relates to a brake booster device for use in a brake system for vehicles.
2. Discussion of the Related Art
A brake booster device of this kind is disclosed in Japanese Patent No. 2967378 issued on Oct. 25, 1999. In this known device, the interior of a booster shell is divided by a diaphragm into a variable pressure chamber and a constant pressure chamber, and the diaphragm is moved by the force of a return spring urging the diaphragm rearwardly and in response to the difference in pressure between the both chambers. The diaphragm is bodily provided with a piston for transmitting the movement thereof in a front-rear direction to an output member which passes through a front wall of the booster shell airtightly. A protruding portion which a rear part of the piston is moved into and out is provided at the center portion of a rear wall of the booster shell to project rearwardly. A cylindrical flexible member like a bellow is tightly fit at its one end into the internal surface of a rear part of the protruding portion and at its other end on the external surface of a rear part of the piston. A retainer is contained in a cave which is provided from the rear end surface of the piston co-axially. An air valve member coupled to an input rod is received in the piston to be slidable relative thereto through a predetermined amount in the front-rear direction. A compression spring is provided for urging the air valve member in such a rearward direction that the air valve portion of the air valve member closes a port which is provided on the retainer for making the variable pressure chamber in communication with the atmospheric pressure. Another compression spring is further provided for urging the retainer in such a forward direction that a negative pressure valve operating portion formed on the retainer closes a negative pressure valve portion which is formed on the piston for making the variable pressure chamber in communication with the constant pressure chamber. Further, a cylindrical portion is protruded from the front end portion of the piston which is exposed into the constant pressure chamber, while a guide cylinder formed on the rear end portion of a master cylinder passes airtightly through the front wall of the booster shell to extend into the constant pressure chamber, so that the piston is guided with the cylindrical portion thereof being snugly fit on the outer surface of the guide cylinder.
However, in the prior art of the aforementioned configuration, the negative pressure valve portion has to be formed within the cave formed in the piston, and the retainer, the air valve member, two compression springs and the like have to be received within the cave. This disadvantageously causes the piston to be larger and complicated in shape and also causes the number of parts to increase. In addition, the spring force of the return spring constitutes an inner resistance, so that the braking power may disadvantageously be lowered when the negative pressure in the constant pressure chamber is lost.